This invention relates generally to methods and apparatus for nondestructively measuring porosity and detecting defects in composite structures.
New product designs and manufacturing methods of structures based on polymer matrix composites are becoming more prevalent. However, composite structures may have an unacceptable level of volumetric porosity and undetected porosity and defects can lead to early failure of components.
One known method for measuring porosity in composite structures is the use of acid digestion. With acid digestion, the weight percent of matrix material and fiber material are measured separately by using acid to dissolve one of the constituents. Using these data plus mass density information for the separate materials, the percent porosity can easily be determined. However, acid digestion methods are destructive because the composite must be dissolved to measure the volume of porosity. Acid digestion is valuable as a process control tool where either entire parts or sections of parts can be sacrificed to measure the capability of the manufacturing process. For some components, however, this destructive testing method is inadequate since actual structures should be measured.
Some known methods for measuring porosity in composite structures made of fiberglass are based on the observation that pores in a composite structure cause a frequency shift in sound traveling through the structure. These known methods use multiple ultrasonic transducers to measure through transmission through a composite article in an immersion tank to determine sound attenuation to estimate the porosity content in composites. In general, these known methods require precision scanning of two transducers in an immersion tank collecting data at a plurality of frequencies. To collect the ultrasonic information needed to analyze porosity would require two or more scans of the part depending on the attenuation slope calculation method used, a serious limitation to manufacturing productivity. Additionally, two transducers are required for these measurements with their positioning axes. However, most immersion tanks designed for such inspection have only one transducer manipulator.
Another known method for measuring porosity of composite structures made of resin infused fiberglass uses a single transducer to determine sound attenuation by the composite structure in an immersion tank. In this method, sound reflected off the front wall of a composite structure in an immersion tank is compared with sound reflected off the back wall the immersion tank after passing through the composite structure. This known method typically uses a 2.25 MHz frequency transducer.
When composite structures are made of materials other than fiberglass, some of the known methods are not as efficacious. When composite structures are very large, testing the large structure in existing immersion tanks according to the known methods may be impractical or impossible.